This invention relates to cemented carbonitride alloys containing titanium (Ti) and tungsten (W), whose cutting property is markedly improved.
Titanium carbide (TiC) base alloys are superior to tungsten carbide (WC) alloys in oxidation resistance and wear resistance when used as a cutting tool, in addition to the low price and light weight of the former.
The TiC base alloys having these excellent properties have been watched with keen interest as a promising material for tools and various alloys of this kind have been proposed, but the scope of the intended use thereof is considerably limited, because of the low toughness, large edge deformation when cutting is carried out at a high temperature and high pressure and tendency of breaking in a cutting operation with a heat cycle such as in an intermittent cutting, as is well known in the art.
It has been considered that the essential difference between the TiC base alloy and WC base alloy of the prior art, in particular, the difference of toughness, is due to that of TiC crystal and WC crystal. For example, it is well known that WC crystal is very excellent in strength and plastic deformation resistance at a high temperature and it is assumed that even if another element such as W is dissolved in TiC, the property of TiC itself is scarcely changed and such a high temperature strength that WC has cannot be given to TiC. Therefore, it is necessary to retain a WC phase in TiC base alloys in order to impart to TiC base alloys strength and plastic deformation resistance similar to those of WC base alloys. The other important property, in particular, wear resistance in cutting steels, increases generally with the decrease of WC phase where the hard phase consists of W, Ti and C or with the decrease of the quantity of W in the hard phase of B1 type (MC phase) due to decrease of the reaction of W and steel. That is to say, the wear resistance deteriorates generally with the increase of the content of W.
In a system of Ti-W-C under normal sintering condition at a temperature of lower than 1600.degree. C., for example, there is only a (Ti, W)C phase (MC phase) corresponding to the crystal structure B1 if the quantity of W is less than the boundary line of (Ti.sub.0.5 W.sub.0.5)C and there are deposited WC phase and MC phase if more than that. Therefore, it is impossible to deposit WC phase if the concentration of W is lowered. In the MC phase, the position of M can be substituted by one or more high melting point metals of Groups IVa, Va and VIa (Periodic Table) and the position of C can be substituted by N.
It is reported by R. Kieffer, P. Ettmayer and M. Freudhofmeier, Metall 25, (1971) p. 1335 that the strength properties of cemented titanium carbonitrides as a tool material for high speed finish of steels can be improved. Competitive carbonitride alloys for high speed finishing of steels and super alloys are only recently achieved through use of a novel decomposition reaction on the system Ti-Mo(W)-C-N, but, because of their low thermal conductivity, these carbonitrides are not suitable for interrupted cuts and milling operations at heavy feed rates.